A power management system is usually employed in an electronic system for managing power supply from a power source. When an over-current condition occurs, the electronic system can be deteriorated. Disadvantages and drawbacks exist in conventional power management systems, e.g., in an over-current protection function. In one conventional topology, the presence of one sense resistor for each DC/DC converter can increase component number and cost, and decrease system efficiency. In another conventional topology, an output current can be detected and information of the output current is fed back to the power management system. Thus, over-current protection actions can be triggered according to the feedback information. However, an unstable output current can cause the over-current protection inaccurate and system noises can be increased.